The present invention concerns a ferroelectric memory circuit comprising a ferroelectric memory cell in the form of a ferroelectric polymer thin film and first and second electrodes respectively contacting the ferroelectric the memory cell at opposite surfaces thereof, whereby a polarization state of the cell can be set, switched and detected by applying appropriate voltages to the electrodes. The invention also concerns a method in the fabrication of a ferroelectric memory circuit of this kind, wherein the memory circuit is provided on an insulating substrate.
The present invention deals with the polarization and switching process in a ferroelectric polymer thin film in memory circuits. Such circuits are used to realize bistable ferroelectric memory devices.
In particular the present invention concerns how to improve the performance of ferroelectric poly(vinylidene fluoride-trifluoroethylene) polymer thin and ultrathin films in a circuit of this kind, where memory cells in the thin film are switched between two polarization states by means of an electric field.
Ferroelectric thin (0.1 xcexcm to 1 xcexcm) and ultrathin (below 0.1 xcexcm) films can be used as bistable memory devices are well-known in the prior art. The use of ferroelectric polymer in thin film form can realize fully integrated devices in which polarization switching can occur at low voltages. However, the investigation of the thickness dependence of polarization behaviour of the most widely used ferroelectric polymer according to prior art, i.e. polyvinylidene fluoride-trifluoroethylene (PVDF-TFE), shows that the polarization level decreases and the switching field increases as the thickness is reduced, and further that a large drop in the polarization level is observed as the thickness is reduced to below 100 nm. In PVDF-TFE polymer films, the polarization behaviour is directly related to the crystallinity and crystallite size in the film. It is believed that in thin films, a stiff metal substrate on which the film is normally deposited by spin-coating, may inhibit the crystallization process due to the heterogeneous nucleation process which determines the crystallite orientation being influenced by the substrate. As a result, neighbouring crystallites may have large orientation mismatches which cause a high elastic energy in the film and prevent the further growth of crystallites, thus creating an interface region between the metal substrate and the thin film. On the other hand, recent experimental results seem to indicate that a high crystallinity may be obtained even with a metal substrate, so that the actual mechanism at present remains somewhat unclear. The interface has a thickness which is a considerable fraction of the thin-film thickness, causing a lower polarization level and a higher coercive field. Due to said interface, thin films in contact with a metal layer exhibit a lower polarization level and high switching field.
Hence, a major object of the present invention is to obviate the above-mentioned disadvantages of the prior art technology for ferroelectric memory circuits. Particularly it is also an object of the present invention to improve the polarization and switching behaviour in ferroelectric memory circuits with ferroelectric polymer thin films as the memory material.
The above objects as well as further features and advantages are realized with a ferroelectric memory circuit according to the invention which is characterized in that at least one of the electrodes comprises at least one contact layer, said at least one contact layer comprising a conductive polymer contacting the memory cell, and optionally a second layer of a metal film contacting the conducting polymer, whereby said at least one of the electrodes either comprises a conducting polymer contact layer only, or a combination of a conducting polymer contact layer and a metal film layer.
In an advantages embodiment of the ferroelectric memory circuit of the invention, wherein only one of the electrodes comprises the conducting polymer contact layer, the other electrode comprises a single metal film layer.
Preferably the ferroelectric polymer thin film has a thickness of 1 xcexcm or less and preferably the conducting polymer has a thickness between 20 nm and 100 nm.
Preferably the ferroelectric memory cell comprises at least one polymer selected among one of the following, viz. polyvinylidene fluoride (PVDF), polyvinylidene with any of its copolymers, ter-polymers based on either copolymers or PVDF-trifluoroethylene (PVDF-TFE), odd-numbered nylons, odd-numbered nylons with any of their copolymers, cyanopolymers, and cyanopolymers with any of their copolymers. In that connection it is preferred the conducting polymer of the contact layer is selected among one of the following, viz, doped polypyrrole (PPy), doped derivatives of polypyrrole (PPy), doped polyaniline, doped derivatives of polyaniline, doped polythiophenes, and doped derivatives of polythiophenes.
Generally it is preferred that the conducting polymer of the contact layer is selected among one of the following polymers, viz., doped polypyrrole (PPy), doped derivatives of polypyrrole (PPY), doped polyaniline, doped derivatives of polyaniline, doped polythiophenes, and doped derivatives of polythiophenes.
It is also preferable that metal of the metal film layer is selected among one of the following, viz. aluminium, platinum, titanium and copper.
Advantageously the ferroelectric memory circuit according to the invention forms a memory circuit in a matrix-addressable array of similar circuits, that memory cell of a memory circuit forms a portion in a global layer of ferroelectric polymer thin film, and the first and second electrodes form portions of a first and second electrode means respectively, each electrode means comprising a plurality of parallel strip-like electrodes with the electrodes of the second electrode means being oriented at an angle, preferably orthogonally, to the electrodes of the first electrode means with the ferroelectric polymer thin film global layer in sandwich therebetween, such that the ferroelectric memory cell is defined in the ferroelectric polymer thin film at the crossings of respectively the electrodes of the first electrode means and the electrodes of the second electrode means, whereby the array formed by the electrode means and the ferroelectric polymer thin film with the memory cells forms an integrated passive matrix-addressable ferroelectric memory device wherein the addressing of respective memory cells for write and read operations take place via the electrodes of the electrodes means in a suitable connection with external circuitry for driving, control and detection.
The above-mentioned objects as well as further features and advantages are also realized with a method in the fabrication of ferroelectric memory circuit according to the invention, the method being characterized by depositing a contact layer of conducting polymer on the substrate, depositing subsequently a ferroelectric polymer thin film on the contact layer, and then depositing a second contact layer on the top of the ferroelectric polymer thin film.
In the method according to the invention it is considered advantageous depositing a metal film layer on the substrate before the first contact layer is deposited and depositing the latter subsequently.
In the method according to the invention it is preferable depositing the conducting polymer thin film by means of spin coating, and similarly depositing the ferroelectric polymer thin film on the first contact layer by means of spin coating.
In a preferred embodiment of the method according to the invention the first contact layer and/or the ferroelectric polymer thin film are annealed at a temperature of about 140xc2x0 C. after the respective deposition steps.
In another preferred embodiment of the method according to the invention a second contact layer of a conducting polymer thin film is deposited on the top of the ferroelectric polymer thin film. In that connection it is preferred to anneal the second contact layer at a temperature of about 140xc2x0 C. without annealing the ferroelectric polymer thin film before depositing the second contact layer, and preferably a metal film layer can be deposited on the top of the second contact layer.